Modulation of electrical quantities



F. J. D'AGOSTINO ET AL 2,385,086

MODULATION OF ELECTRICAL QUANTITIES Filed April 19, 1943 2 SheetsSheet 2SOURCE M PHOTO/U55 M/F/PO/F O/SFZLHC 1 hire??? [warm Sept. 18, 1945.

m m WM H w ME 5 F 2 \M w Em I A\ mruw Wm W +r 5 5 55 NP r r z i i I 5;m5 r m /-afl M fill I p u Patented Sept. 18, 1945 UNITED STATES PATENTOFFICE Valdes, Buenos Aires, Argentina assignors to Hartford NationalBank and Trust Company, Hartford, Conn., as trustee Application April19, 1943, Serial No. 483,646

9 Claims.

The present invention relates to the modulation of electricalquantities, and more particularly, though not restrictively, to themodulation of oscillating quantities such as are used in frequency orphase modulation in radio transmission.

In the production of frequency modulated waves and in various otherbranches of radio technique, the problem frequently arises ofgenerating, from a signal u(t), which may, for example, appear as atension reproducing some kind of intelligence, including integratedintelligence, another signal which shall vary as the sine or cosine ofan angle 1 proportional to the former. The problem becomes particularlydifficult when the angle 1' to be obtained is of the order of tens orhundreds of radians.

The present invention provides for this problem a substantially completesolution based on the electrical control of a system of opticalinterference and the conversion into electrical terms of the resultsobtained, by the aid of a photoelectric cell.

It is known that, if two rays of coherent. monochromatic light, theluminous intensities of which are supposed equal, meet after travellingover two paths of different lengths, optical interference will result.It is well known that the luminous intensity i obeserved at the meetingpoint varies in accordance with the law where J is the common intensity,d the difference in length of the paths, and l the wavelength of thelight.

The fundamental idea of the present invention consists, therefore, incontrolling, by the signal u(t), the difference in length of pathtraversed. in such wise that said difierence in length d shall beproportional to the signal u. The factor of proportionality, h, betweend and u, is a constant for any given set of conditions. From this itfollows that the luminous intensity at the meeting point will be equalto 2J(1+cos l'), where i =kit(t) and where h=length/tension. That is tosay, the variable portion of this intensity exactly reproduces thedesired cosine.

If, therefore, at the meeting point of the interfering rays, aphotoelectric cell is placed which gives a current proportional to theluminous intensity 9 present, and has a given coeflicient of sensitivityA amperes per lumen, say of the order of aA/lumen, such current may bepassed through a suitable resistance R and the variable portion only ofthe tension thus generated may be applied to the grid of an ordinaryvalve,

10 whereupon the desired tension v=V cos l' is obtained on this grid,with V=2JAR.

For the better understanding of the present invention the same will nowbe more particular- 1y described .with reference to the accompanying l6drawings, in which Fig. 1 is a diagram illustrating the practicalapplication of the principles of the invention.

Fig. 2 is a diagram illustrating an alternative method of embodying theinvention,

Fig. 3 is a block diagram illustrating an application for the novelmodulating method.

Fig. 4 diagrammatically illustrates apparatus in accordance with anotherembodiment of the invention;

28 Fig. 5 is a diagram showing an interference pattern and thepositioning of photocathodes for producing an output tension inaccordance with the invention;

Fig. 6 is a diagram illustrating apparatus for obtaining conjugatetrigonometric functions proportional to an applied electrical quantity.

Referring to the drawings, I0 is a source of monochromatic light ofwavelength A, I2 is a separator for separating said light into two beamsR and R", which are directed along respective paths, one of whichcomprises a modulator l4 designed to provide a difference in length dbetween the two paths and actuated by a signal u(t) to cause variationsof d proportionately to the variations in said signal. The two beams arecaused to impinge on a collector l6 which is adapted to cause the beamsto meet at a point D where the sensitive element I! of a photoelectriccell 20 is located. The output of said cell is connected as shownbetween the grid 22 and the cathode 24 of a valve 26 which isconveniently the first valve of a standard amplifier.

In Fig. 1 only one modulator has been shown,

but it is to be understood that a modulator may be located in the pathof each of the two beams,

if desired.

The fundamental advantages of the novel method of modulation of thepresent invention are, first, that the sinusoidal law is, in principle,

obtained with complete precision, since it is based on a fully provedlaw of nature. All that is requisite is to realize the "point" ofmeeting, and to make the linear transformations 11-11 and 1-1:.Secondly, the angle l the cosine of which i enerated, may in principlattain values of th ands of radians, since the diflerence in length ofpath d may, in the modulators to be described hereinbelow, reach valuesof up to a millimeter. If violet light be used having a wavelength of0.-i =4 10 mm., which is the zone in which the photoelectric cell hasits maximum response in certain cases, the maximum angle may beapproximately equal to 21Xfi=16fl00 radians Hence this novel methodoilers a flexibility in modulation more than sufficient for allpractical applications.

In practice, the systems which divide the rays of light emerging from asource into two beams and subsequently reunite them, are such that, atthe several points of plane located around the point of union, therealways arrive pairs of rays coming from diflerent paths and havingvarious differences of travel. Hereinabove, mention has been made of the"point" of union, but such an abstraction can never be realised.Actually, there are only bands of interference having a certain width.The luminous condition remains identical with itself only along everyideal straight line of the plane onto which the bands are prolected.Hence, in order to approximate as closely as possible the said point ofmeeting, matters must be so arranged that bands are formed of thegreatest possible width and the photosensitive element must be locatedalong a narrow strip inside the bands, for instance along the centerlineof such a band. The generation of a light pattern comprising a pluralityof interference bands and the positioning of photosensitive elementsalong a narrow strip inside the bands has been shown in Fig. 5. Theforegoing remarks imply that the interference optical system must be ofthe type giving wide bands, and, as the luminous state is more definitethe narrower the "line" to which it is referred, that the outputefiiciency of the photoelectric cell shall be inversely proportional tothe precision with which the abstract sinusoidal law is formed.

The second, last-named limitation is of no practical importance, as,even if the cell is fitted with very fine exploring elements, thesensitivity may be increased at will by well-known and readilyapplicable radio-electrical means. It is also possible to project on theplane of the cathode of the photoelectric cell, a system ofinterferences comprising more than one band and to locate photosensitiveelements along the center-lines of successive bands of the same kind.

The limitation as to the necessary width of the bands is more important.Systems such as the bi-laminar arrangements of Jamin or Michelson, givebands the width of which is inversely proportional to the angle formedby the laminae. A width of band of the order of some millimeters, whichallows of the definition of a luminous state by an element which can yetbe readily manufachired and of width of some tenths of a millimeter,corresponds, with light of 0.4 u. to angle of the order of hundredthsand less, the maintenance of which is a matter of delicacy when thedifference of length d of the path has to be modulated by thedisplacement as a whole of the laminae. Thus, the conditions for thewidth of the band and for the amplitude of the modulation contradicteach other, and the compromise between the two constitutes thefundamental limitation of the present method. In spite of this. however,the variation in the angle q which may be attained is, in principle, solarge that the applications hitherto projected would be amply satisfiedby a readily achievable compromise.

It will be understood that the source III will be of some extent, andthis will precisely admit of the use of interferential systems workingwith parallel rays, from which th system to be utilised in theperformance of the present method has to be selected. The rays" arereally parallel beams the width of which would be defined by suitableslits or screens.

The modulation properly so called, which consists in varying the lengthof path proportionately to a tension, can be eflected by means of anyelectro-mechanical process. If, for example, a bilaminar interferometerof Jamin or Michelson is used, it is sufiicient if one or both thelaminae are made of piezo-electric material such as Rochelle salt, andthe tension representing the intelligence is applied between the facesthereof. The thickness of the lamina itself in the direction in whichthe light passes through it is then the len th which is varied. with aset of cemented laminae of the bichrome type, displacements of somemicrons per volt are generally achieved. It is also possible to selectan interferometer one of the elements of which is a mirror and to movethe mirror with the rhythm of the intelligence by means of a relay. Suchan arrangement is shown in Fig. 4 wherein a light ray derived from asource 50 impinges on a semi-transparent mirror II and is transmitted toa displaceable mirror 52 and a fixed mirror 58. The ray impinging on thedisplaceable mirror 52 is reflected and transmitted through the element5| to the cathode 5! of a phototube 54. The ray impinging on the fixedmirror BI is redirected towards the element H from which it is reflectedto the cathode SI of the phototube. Light interference efl'ects areproduced at the surface of the cathode 86 by movement of thedisplaceable mirror 52 which varies the length of path of one light raywith respect to the other light ray impinging on the cathode. Forenergizing the displaceable mirror there are provided an armature 51 onwhich the mirror is mounted and an exciting coil for the armatureenergized by the modulating source 59. There are so many possiblemethods available and known to those skilled in the art of optics, thatfurther description thereof is deemed superfluous. The sensitivity ofsuch apparatus may readily reach tens of microns, and, with light-weightmirrors, up to a millimeter per volt. The frequency is limited by thatat which the displacements and mechanical deformations can take placewithout excessive power consumption, that is to say, in general, forsmall powers of the orders of deciwatts or watts, about 15,000 c./s.

The difficulty caused by the limited width of the areas which, inordinary inference systems. have a uniform luminous state, does notpresent itself in those systems where the interference takes placebetween the two rays, ordinary and extraord na y. which issue from somebirefringent crystals. It is known from the art of crystalline opticsthat, if a lamina of a crystal presenting the two indices of refractionn and n", is placed between two crossed polarizers (nichols), the firstone being illuminated by a parallel beam of monochromatic light ofintensity J, the whole volone behind the second nichol is filled with auniform luminous state its intensity being where L is the length ofcrystal traversed and x the wave-length of the light. In this manner thewhole area of a photocathode can be illuminated and the necessity ofhyperfine sensitive elements avoided. As the square of a cosine reducesto half the sum of unity and the cosine of the double angle, it is seenthat, with a photocell of the properties indicated hereinabove(sensitivity A and plate resistance R), the variable, i. e., usefulpart, of the electrical tension generated is The quantity 9 is modulableas before. by an intelligence Mt), but the modulation does not nowdispose oi more than the lamina where the interference originates. Here.either the magnitude L. or the magnitude 7 n"n' that is the doublerefraction indices, may be operated on. Modulation of the length Lsupposes the use of a crystal presenting an electro-mechanical effect,but these crystals. unfortunately, possess in general a poorbirefringence, i. e, a very feeble value of The good birefringentmaterials have indices up to the order of 0.1, whereas those which aresimultaneously piezo-electric do not attain values higher than 0.02 insome samples of Rochelle salt. This naturall reduces the magnitude ofthe angle q which can b created. Since the Rochelle salt and the nicholsare transparent up to the wave-length of 0.15 a region where goodphoto-electric cells may still be found, ultra violet light up to saidwave-length may be used in order to improve the sensitivity. With anelectro-mechanical constant length tension of 1 micron per volt. such asthat offered by the natural Rochelle salt, the technique of bichromelaminae being excluded here, and an intelligence of 100 volts, a valueof w of 21 o.o2 =85 radians can be created. This is evidently less thanthat given by the "isotropic" interference systems already discussed.but still constitutes an improvement of 85 times upon the performance ofknown methods of purely electrical phase modulation. On the other handand in exchange for this drop in range, the device for realizing the"anisotropic" interference topics now being discussed is particularlysimple. Fig. 2 shows that it reduces to a source Ila, a lens 20 forconverting the light into a parallel beam H, the crossed polarizers 8|,ll on either side of a birefringent and piezo-electrio lamina I2 and acommon photoelectric de vice "a, without any special cathodeconstruction, from the anode 3| of which the useful current is obtained.The intelligence uit) is applied to the lamina in the same manner asbefore.

Another way of modulating the angle created by the anisotropic deviceswould consist in varying the magnitude 1 instead of the length L. Thisleads to the use of media whose index of double refraction depends on anelectrical or magnetic fleld, and which are typically represented bynitrobenzine with its well-known Kerr effect. This idea and method havebeen disclosed and claimed in the prior United States patent applica- 5tion No. 450,596, flied July 11, 1942, in the name of Edouard Labin. '1Q e pr mnhmethod ag etdi g fre quency'modulation or phase modulation, by

following the lines laid dowiiawnearoriam "i? prior patent applicationSer. No. 450,596, since, all that is required is that it shall bepossible to develop the sine as well as the cosine of the same angle P.Th i, as will be evi dgnii canreadlly be achieved in theisotropwiiiterferencesystemiby' i5 locatin'gfsensitlveelements"of'photoelectric cells along the center liners: co'nJugatebands ofinterference, so that when the luminous intensity in one band issin'Tthat in'tlTeneighbouring band is jc'os'q'i arrangement inconformance'with the above is shown in Fig. 6. As appears from Fig. 6,light from a source It passes through a separator I! which divides thelight into two rays. In the path of one of the rays is a modulator Hwhich varies the length of path of the ray therethrough upon actuationby a signal u(t) applied thereto.

By means of a collector it the two rays are directed towards twophototubes l0 and I and form a plurality of interference bands in theplane of the photosensitive cathodes l2 and H of so the phototubes. Thecathodes 62 and i! are so positioned relative to each other that onecathode is subjected to maximum light intensity of the interferencepattern when the other cathode is subjected to minimum light intensity.The

function cos P and sin a are derived from the output electrodes '4 and8! of the phototubes I and Ii as shown.

The sensitive elements located in corresponding interference bands willbe connected to an 4 output giving say, the cosine function, while thosestrips located in corresponding bands conjugate to the former, will beconnected to an output giving the sine function, all the strips beingcapable of arrangement in a single photoelectric unit or in individualphototubes as shown in Fig. 6. g

In the anisotropic method. duplicate optical systems will have to beemployed, arranged in quadrature with each other. This condition may bemet by having the polariser and analyser crossed in one system andparallel in the other.

or else by having both the polariser and the analyser crossed in the twopaths, one set being rotated through 90 with respect to the other.

Fig. 3 illustrates schematically the arrangement for obtaining a wave ofthe type A sinunt-ll') by the addition of two components (sin not. cos iand (cos mt. sin 9) obtained by mixing a sinuso'dal ilot centraloscillation with the appropriate sinusoidal component obtained from theintelligence by modulation according to the method of the presentinvention. The pilot oscillation is generated in the oscillator I andapplied to two phase shifting means I! and 42'', the wave derived fromthe one phase shifting means being in quadrature with that derived fromthe other phase shifting means. A modulator No as hereinabove describedis provided for producing a pair of oscillations which are sinusoidalfunctions in quadrature of the angle related to the intelligence, andone of said oscillations is mixed with the corresponding pilotoscillation in mixers ll, 44" to form the products sin vet. cos? and cosvet. sin *1, the outputs of said mixers then being appliedsimultaneously to a common 75 point of the utility circuit.

L Yo 1 L e apn edj It will be clear to those skilled in the art that thepresent method may be usefully and beneflcially employed in connectionwith other applications, and that we may make sundry modiflcatlons inthe details hereinabove disclosed without thereby departing from thespirit and scope of the present invention as defined in the appendedclaims.

We claim:

1. A method of generating an electrical quantity equal to a sinusoidalfunction of an angle proportional to a useful electrical magnitude,which comprises the steps of generating two component light beams ofconstant and equal luminosity, directing the said beams along separatepaths, varying the effective length of the optical paths of the beamsrelative to each other by an amount greater than three-fourths of awavelength of the light of said beams and proportional to the variationsof said useful electrical magnitude, interfering the light energies ofsaid beams to produce combined light energy having a luminosity varyingas a sinusoidal function of said phase difference, and transforming saidcombined light energy into electrical energy to produce an electricalquantity varying as a sinusoidal function of said phase differenceangle.

2. A method of generating an electrical quantity equal to a sinusoidalfunction of an angle proportional to a useful electrical magnitude,comprising the steps of generating a beam of monochromatic light energy.dividing said beam into two component beams of equal luminosity,directing said component beams along separate paths, varying theeffective optical length of at least one of said paths by an amountequal to a plurality of wavelengths of the light energy and proportionalto the said electrical magnitude, interfering said component beams toproduce combined light energy having a luminosity varying as asinusoidal function of an angle proportional to the length of themodulated optical path, and transforming said combined light energy intoelectrical energy to produce an electrical quantity varying as asinusoidal function of said angle.

3. A device for generating an electrical quanti y equal to a sinusoidalfunction of an angle proportional to a useful electrical quantity, whichcomprises means for generating two monochromatic light beams of constantand like luminosity. means to direct said beams along separate paths,means to vary the effective optical length of one of said paths by anamount greater than threefourths of a wavelength of the light of saidbeams and proportional to said useful electrical magnitude to produceoptical phase differences between said beams proportional to saidelectrical magnitude, means to combine said beams on emergence from saidpaths to produce an interference pattern of combined light energycomprising bands having a luminosity varying as a sinusoidal function ofan angle proportional to the optical length of the modulated path. andphotosensitive means to transform the light energy of one of saidvarying luminosity bands into electrical energy.

4. A device according to claim 3, in which the optical path varyingmeans is an element of piezoelectric characteristics.

5. A device according to claim 3, in which the optical path varyingmeans is a mirror displaceable in the direction of the correspondinglight beam in response to said useful electrical quana,sas,oso

6. A device according to claim 3, in which said light beam combiningmeans are arranged to produce an interference pattern having relativelywide bands, the said photosensitive means bein provided with a pluralityof photosensitive elements each located along the center line of similar7. A method of generating an electrical quantity equal to a sinusoidalfunction of an angle proportional to a useful electrical magnitude,which comprises the steps of generating a beam of monochromatic light,polarizing said beam. dividing the polarized beam by double refractioninto two virtual component beams, varying the optical path lengthdifferently for each virtual beam by an amount greater than threefourthsof a wavelength of the monochromatic light and proportional to saidelectrical magnitude to produce an optical phase difference between saidvirtual beams proportional to said electrical magnitude, subjecting saidvirtual beams simultaneously to polarization in quadrature with thefirst mentioned polarization to protime an emerging interference beamthe luminous intensity of which varies sinusoidally with said phasedifference angle, and transforming said emerging beam into electricalenergy.

8. A device for generating an electrical quantity equal to a sinusoidalfunction of an angle proportional to a useful electrical magnitude,which comprises means for generating a beam of monochromatic light ofconstant intensity, means to polarize the beam, an anisotropic sectionto split the polarized light beam into two virtual component beams andconsisting of a member of crystalline, double refracting plezo-electrlcmaterial, means to vary the optical length of said anisotropic sectionbyan amount greater than three-fourths of a wave-length of themonochromatic light and proportional to said electrical magnitude, andan analyzer to combine said virtual component beams to produce aconversion zone of lnterferential luminous intensity. said conversionzone comprising photosensitive means to convert the interferential lightenergy of said zone into electrical energy varying as a sinusoidalfunction of said electrical magnitude.

9. A method of generating two electrical quantitles representingconjugate sinusoidal functions of an angle proportional to a modulatingpotential, comprising the steps of generating two component light beamsof constant and an equal luminosity, directing said component beamsalong separate paths, varying the effective length of one of said pathsrelative to the other by an amount greater than three-fourths of awavelength of the light of said beams and proportional to saidmodulating potential, interfering said un-moduiated and modulatedcomponent beams to produce an interference light pattern comprising atleast two bands each having a luminosity decreasing and increasing,respectively, as two conjugate sinusoidal functions of an angleproportional to the length of the modulated optical path, andtransforming the light energy of each of said bands into electricalenergy to produce a first electrical quantity varying proportionally toa sinusoidal function of said angle and a second electrical quantityvarying proportionally to the con- 70 iugate trigonometric function ofsaid sinusoidal function.

, FRANCISCO J. D'AGOB'IINO.

B. A. VAL-DEB.

